It's true that NetBSD has a Linux compatibility layer, but it is only aimed at running Linux binaries on NetBSD, it won't help running anything on MacOS X. Except of course if you port this Linux compatibility layer to MacOS X.

Even if you do that, Mac-on-Linux is based on a kernel module, and because they just emulate the kernel behavior, compatibility layers can only run userland programs. You could immagine porting the Mac-on-Linux kernel part to NetBSD or MacOS X, though. There is nothing extremely difficult there.

But porting Mac-on-Linux to MacOS X seems a very useless work, as MacOS X already has the ability to run older MacOS programs through the classic environnement.